In the art of dentistry, a dental handpiece is attached to and driven by a motor, of which rotation is transmitted through a train of gears to a dental tool, such as a cutting bur, at the tip of the handpiece for cutting teeth. There are a variety of trains of gears, such as for multiplying the rotation speed up to five times, transmitting the rotation at a direct ratio, or reducing the rotation speed. For example, in a handpiece having a gear train increasing the speed up to five times, the rotation speed of a motor at 40000 rpm is increased into the rotation speed of a bur at as high as 200000 rpm (five times) for cutting teeth. With a dental tool rotating at as such a high speed as 200000 rpm, proper maintenance, such as cleaning or lubricating, of the dental tool keeps the handpiece from abnormal heat generation, but absence of proper maintenance may result in attachment of dust or contaminants, or lubricant shortage in the driving section of the handpiece, including gears and bearings, which may cause abnormal heat generation.
In order to protect a dental handpiece from abnormal heat generation, a control technique is proposed, for example, in JP H08-308858. This publication discloses an electrical motor controlling system having a protection mechanism which warns that the current work is overloading when the micromotor is loaded so heavily as to generate heat to threaten its motor life, but still allows normal operation when the overloading is ceased.
Such an electrical motor controlling system detects the load of the micromotor during actual treatment of a patient, i.e., the load during tooth cutting, generates warning, and operates to suppress heat generation by the micromotor. However, this system cannot inform the user, before starting the operation, whether the handpiece parts connected to the micromotor are properly maintained, or the handpiece is appropriate for use, so that the user cannot decide whether to do the maintenance of the handpiece or not.